1. Field of the Invention
The present invention relates to a wide dynamic range image capturing apparatus for processing video data supporting an image sensor developed for a digital camera.
2. Description of the Related Art
In recent years, the camera industry is remarkably moving from analog technology to digital technology. Particularly, a boom has occurred in digital still cameras, which require neither films nor development, and mobile telephones with built-in digital camera are becoming mainstream mobile telephones.
At present, most digital cameras adopt signal processing which supports a sensor having a primary color filter, in favor of color reproduction. There is a demand for a signal processing technique for digital cameras which can provide excellent color signal-to-noise ratios while putting importance on resolution, and can also provide excellent tone characteristics and high sensitivity.
As a technique in such a field, for example, Japanese Unexamined Patent Application Publication No. 63-306779 discloses a technique of controlling the exposure amount (electric charge accumulation period) of an image capturing device and successively combining images, thereby making it possible to support moving images.
FIG. 7 is a block diagram illustrating a configuration of a conventional wide dynamic range image capturing apparatus. In FIG. 7, 201 indicates an analog-to-digital converter, 202 indicates a control circuit, 203 indicates a digital-to-analog converter, 204 indicates a camera signal processing circuit, 205 indicates an image capturing device for performing photoelectric conversion with respect to a subject, 206 and 207 indicate field memories, and 208 indicates a switch circuit. Hereinafter, an operation of the conventional wide dynamic range image capturing apparatus will be described.
Initially, in the image capturing device 205, images are successively captured while switching the electric charge accumulation period between 1/1000 seconds and 1/120 seconds (see (a) of FIG. 8). Here, an image captured when the electric charge accumulation period is 1/1000 seconds is temporarily stored into the field memory 206.
Next, with timing of an image obtained during the electric charge accumulation period of 1/120 seconds, the image stored during the electric charge accumulation period of 1/1000 seconds is read from the field memory 206 (see (b) of FIG. 8). Thereafter, the image is written into the field memory 207 while switching (combining) the electric charge accumulation period between 1/120 seconds and 1/1000 seconds, depending on a predetermined condition. Note that it is assumed that, when there is overexposure during the electric charge accumulation period of 1/120 seconds and there is not underexposure during the electric charge accumulation period of 1/1000 seconds, an image of 1/120 seconds is switched to an image of 1/1000 seconds. Thus, in the control circuit 202, images are switched (combined) (see (c) of FIG. 8).
Thereafter, the combined image signal is subjected to speed conversion in the field memory 207 so that a scanning speed is returned to the speed of a normal TV signal, which is in turn output ((d) of FIG. 8).
By performing such an image combining process, an image which is otherwise conventionally overexposed or underexposed is replaced with an image having a different electric charge accumulation period, thereby making it possible to enlarge the dynamic range of images.